Colorectal cancer is a major cause of cancer death. Morbidity, mortality and healthcare costs can be reduced if the
disease can be detected at an early stage. Screening is a viable approach as there is a clear link to risk factors such as
age. We have developed a fluorescent contrast agent for use during colonoscopy. The agent is administered
intravenously and is targeted to an early stage molecular marker for colorectal cancer. The agent consists of a targeting
section comprising a peptide, and a fluorescent reporter molecule. Clinical imaging of the agent is to be performed with
a far red fluorescence imaging channel (635 nm excitation/660-700 nm emission) as an adjunct to white light colonoscopy. Preclinical proof of mechanism results are presented. The compound has a K<sub>d</sub> of ~3nM. Two human xenograft tumour models were used. Tumour cells were implanted and grown subcutaneously in nude mice. Imaging using a fluorescence reflectance imaging system and quantitative biodistribution studies were performed. Substances tested include the
targeted agent, and a scrambled sequence of the peptide (no binding) used as a negative control. Competition studies were also performed by co-administration of 180 times excess unlabelled peptide. Positive imaging contrast was shown in the tumours, with a clear relationship to expression levels (confirmed with quantitative biodistribution data). There was a significant difference between the positive and negative control substances, and a significant reduction in contrast
in the competition experiment.
Colorectal cancer is a major cause of cancer death. A significant unmet clinical need exists in the area of screening for
earlier and more accurate diagnosis and treatment. We have identified a fluorescence imaging agent targeted to an early
stage molecular marker for colorectal cancer. The agent is administered intravenously and imaged in a far red imaging
channel as an adjunct to white light endoscopy.
There is experimental evidence of preclinical proof of mechanism for the agent. In order to assess potential clinical
efficacy, imaging was performed with a prototype fluorescence endoscope system designed to produce clinically relevant
images. A clinical laparoscope system was modified for fluorescence imaging. The system was optimised for
sensitivity. Images were recorded at settings matching those expected with a clinical endoscope implementation (at
video frame rate operation). The animal model was comprised of a HCT-15 xenograft tumour expressing the target at
concentration levels expected in early stage colorectal cancer. Tumours were grown subcutaneously. The imaging agent
was administered intravenously at a dose of 50nmol/kg body weight. The animals were killed 2 hours post
administration and prepared for imaging. A 3-4mm diameter, 1.6mm thick slice of viable tumour was placed over the
opened colon and imaged with the laparoscope system. A receiver operator characteristic analysis was applied to
imaging results. An area under the curve of 0.98 and a sensitivity of 87% [73, 96] and specificity of 100% [93, 100]
Several mechanisms of action can be employed for a molecular imaging contrast agent for use with endoscopy.
Targeting of cell surface molecules that are up regulated at an early disease stage, with a fluorescent labelled vector is
one attractive approach. However, it suffers from the inherent limitation that the concentration of agent available is
fundamentally limited by the concentration of receptor molecules available. Simple models indicate that for successful
imaging with a targeting approach, the imaging system should be able to adequately image concentrations in the
nanomolar region. Such low reporter molecule concentrations have implications for the choice of contrast agent. Target
tissue size and location, the tissue native fluorescence contribution, the brightness of the reporter molecule, and
photobleaching thresholds are all factors which contribute to the choice of reporter. For endoscopic imaging of
millimetre sized target tissue volumes close to the surface Cy5<sup>TM</sup> (650-700nm) wavelengths are preferable to Cy3<sup>TM</sup>
(550-600nm) and Cy7<sup>TM</sup> (750-800nm).
We have constructed a system optimised for sensitivity by tailoring light delivery, collection, filtering and detection, in
order to address the fundamental technical performance limits for endoscopic applications. It is demonstrated through
imaging system calibration, phantom based measurement and animal imaging data that low nanomolar concentrations of
Cy5 based fluorescent contrast agent in millimetre sized superficial lesions are adequately imaged with a clinically
relevant endoscope system in real time. It is concluded that targeting is a technically viable approach for endoscopic
Accurate calculation of internal fluence excited in tissue from an optical source can be used for predicting the performance of fluorescent contrast agents for clinical applications. Solutions of excitation fluence for a steady-state Monte Carlo model and a finite element implementation of the 3d diffusion equation have been compared up to depths of 20mm from a point source located on top of a homogeneous cylindrical phantom for a range of reduced scattering-to-absorption ratios. Differences between the fluence calculated by Monte Carlo and diffusion model is found to be dependent on the transport mean free path (mfp), size of the phantom in relation to the penetration depth, distance from the source and mesh resolution. The differences are small at depths ~ mfp and peak at depths ~2mfp. The differences should ideally reduce to zero at large depths but the magnitude of the differences tend to increase due to the finite boundary in the diffusion model. As an example, for a mfp = 0.817mm similar in magnitude to mesh resolution, diffusion fluence at 1mm, 2mm, 10mm and 14mm is 76%, 59%, 66% and 63% respectively of Monte Carlo fluence. For large mfp's characteristic of non- diffusive regimes, diffusion model overestimates fluence at distances less than one mfp. This work demonstrates that mean free path and mesh resolution are the critical parameters that distinguish the performance of Monte Carlo and diffusion models to define error margins that could be utilized for predictive assessment of imageability of fluorescent agents using the diffusion model.
Frequency dependent attenuation is a pronounced feature of ultrasound propagation in human tissues. A new technique for calculating that parameter from backscattered echoes is demonstrated, and it is shown how the information may be incorporated into a gray scale image. Analysis of the noisy nature of attenuation estimates from conventional backscattered echo signals suggests a new technique for producing less erratic estimates, by manipulating zeroes in the complex frequency domain. No signal averaging is needed, and the method lends itself to analysis of short data segments, thereby providing suitable input for attenuation imaging. Rf data is required. The new method is found to considerably reduce the variance of the attenuation estimates from short data segments. It is found that attenuation-weighted B-mode images are an informative way to show results. The method of zero manipulation, presented here, for producing less noisy pulse-echo attenuation estimates represents a powerful approach towards the problem.
A method for measuring the directivity function of transient fields with a new type of hydrophone that can be located at any convenient distance from the transducer is presented. Fields from planar and focused transducers, for both continuous wave and pulsed excitation, are measured via the new method, and the results compared against conventional measurements as well as against theoretical predictions. The directivity function for pulsed fields is best expressed as a complex directivity spectrum, and images of this fundamental transducer field characteristic are shown to encode a number of unexpected features. The definition and measurement of the directivity function, is not dependent on continuous wave or far-field conditions, and laboratory implementation of the theory is via a new type of hydrophone, with some unusual properties. It is concluded that precise and unambiguous measurement of transducer directivity patterns are straight forward to perform provided a relatively simple, but novel, technique is used. Images of the informative directivity spectrum may be obtained with ease.
This communication presents a new method for measuring the thickness of thin membranes with pulse-echo ultrasound. The method is based on a consideration of the structure of the interference of the two echoes from the sides of the membrane. Essentially, it is the interference between these two echoes that gives rise to poor thickness estimation. The new method, which is explained in detail, proceeds from a consideration of the zeros of the echo signal in the complex frequency domain. Measurements with the new technique are compared with two other methods: the time-separation of echo envelopes, and a cross-correlation method. The analysis is presented with both simulated and real (laboratory) data. The effect of noise is taken into account in the laboratory data. This new method is shown to be capable of measuring sub-wavelength membrane thicknesses with excellent precision. The ultrasound rf signal is required, but a substantial improvement over existing techniques is gained.
A new method for computing images of transient ultrasound fields from transducers of arbitrary shape is developed. For simplicity, only transducers with axial symmetry are considered here, but the extension to square and rectangular radiators is straightforward. The more general case may be treated by the same methods. The method is based on the use of the directivity spectrum -- which can be shown to be a generalization of the angular spectrum. It is ideally suited, however, for application to transient fields, and the formalism contains no evanescent waves. Images of pulses over extensive ranges from a variety of transducers are shown. In particular, it is shown that the transient field from a strongly-focused bowl transducer may be readily calculated, without the approximations that are necessary when using the traditional Tupholme-Stepanishen method. The simulation method is powerful and computationally efficient. It is considerably faster than methods used up to now, and may be applied to the computation of fields that are problematic for standard methods. The final output shown here is a high-quality 'snapshot' of the field, at various distances from the transducer face. Phase of the field is shown.
Some aspects of the extremely complex structure of the pulses utilized in medical ultrasound pulse-echo imaging devices are investigated. The following features are examined, in many instances from a fresh point of view: diffraction, Fourier representation, directivity spectrum, propagation of pulse projections, causality, superluminality, and header wave component. Analytical results are underpinned by experimental findings with (single-element) wideband ultrasound transducers. The discussion is conducted within the context of linear fields.
The development of an exact and practicable technique that allows the full time-space complexity of a wideband ultrasound field to be imaged, from a single set of local measurements, is presented. The method is an improvement over the angular spectrum technique, provides a novel approach towards eliminating evanescent waves, and also allows for more efficient computation of transient fields. The new representation of a field has simply propagating components that may be directly measured (via a specifically designed hydrophone) as the time-varying spatial projections of the field. Reconstruction of the transient field is performed by a technique that is a generalization of the Fourier slice theorem. The theory is vindicated by explicit demonstrations with measurements of the fields from typical ultrasound transducers. Visualization of the field is either as a three- dimensional pressure distribution at any temporal instant, or as the time-variation of the pressure over any plane orthogonal to the field propagation direction. This new method for ultrasound field measurement, prediction, and visualization is shown to be eminently practicable and represents a substantial improvement over conventional methods.
The development of a relatively rapid, but accurate, technique for charactensing pulsed fields from array transducers is reported. The approach enables direct confirmation of the effectiveness of many single element Iarray design, construction and activation procedures. The techniques rests on the employment of a PVDF hydrophone of novel design. Whereas conventional approaches derive field characteristics from point measurements, the new hydrophone allows a direct measurement of the fields 'directivity spectrum' — which efficiently generalises the angular spectrum approach to wideband pulses. The directivity spectrum is shown to encapsulate significant features of both near and far field output characteristics, as well as tightness of focus, even though all measurements are conducted at any convenient distance from the transducer face. The new method is demonstrated in the context of measurements of the fields from typical medical ultrasound transducers. The following field and transducer characteristics are shown : directivity, acoustic axis direction, effective transducer/field coherence, tightness of focus, effective radiating area, effective apodisation, and element uniformity. The relative simplicity of the technique is not compromised when measuring angle-emission characteristics. The theoretical basis for the new field measurement technique is presented, and its advantages over the more usual angular spectrum approach with point measurements, are also discussed. Keywords: Ultrasound; Transducer characterisation; Field characterisation; Directivity spectrum; Large aperture hydrophone.
The aim of the investigation reported here is to clarify the way in which spectral-modifying artefacts, such as tissue attenuation, compromise pulse-wave Doppler measurements, and to accurately measure the magnitude of the corrupting influence of attenuation under controlled laboratory conditions. A theoretical description of the structure of the pulse-echo sequence from a moving scatterer field is constructed from first principles by utilizing a time-domain description of the Doppler process. It is demonstrated that the essential features of the echo signal may be rather more accurately described by a wavelet-, rather than by a Fourier-, transform, and that the power spectrum of the Doppler signal does not necessarily encode the range of the scatterer velocities present in the (pulse-echo) sampling volumes. The analysis provides a better understanding of the origins of the significant levels of noise present in pulse-wave Doppler signals, and allows a novel approach towards noise-reduction -- by zero manipulation in complex frequency space -- to be developed.
The specific problem of compensating for the presence of speckle in ultrasound B-mode imaging systems is addressed. The basic principles underlying current approaches to speckle reduction are first briefly reviewed. For real-time imaging systems, speckle reduction techniques which operate in the context of a single image (or `look') of the object are of more practical importance. A large number of such techniques have been developed which are based on some form of noise model of speckle, and are hence statistical in nature. All such approaches reduce the presence of speckle at the cost of some other factor of image quality, usually resolution. This paper examines an alternative approach to the single image problem, based on an alternative speckle definition, which does not suffer the usual resolution trade-off. The basis of the approach is to reformulate the single image problem in terms of a structure uniqueness problem which underpins a speckle model based on zeros of the analytic continuation of the rf A-line signal into the complex time and frequency planes.
Ultrasound pulses utilized for medical imaging and information-gathering appear to be coherently scattered from the many inhomogeneities within a tissue. The consequent interference effects complicate the spectral (Fourier) domain properties of the received signal: this is particularly troublesome when attempting to estimate tissue attenuation from backscattered data. A novel way to describe interference effects in (ultrasound) pulse-echo data is described. Recognition of their influence is achieved via analysis of the temporal phase of the pulse-echo signal, and correction of the artefact is achieved via novel signal processing techniques which rely on adjusting the locations of dominate zeros of the analytic continuation of short corrupted data segments (as pinpointed by the recognition procedure) into the complex frequency domain. Estimation of the mean frequency of a short pulse-echo data segment by the new method gives a reduction in variance by a factor of > 4 over existing Fourier methods. The technique finds application in an imaging technique which incorporates ultrasound attenuation information in conventional B-mode imaging.